1. Field of the Invention
This invention generally relates to a surface acoustic wave device, and more particularly, to a surface acoustic wave device, which is employed in an RF unit on a mobile telephone and features an excellent low loss and an excellent shape factor.
2. Description of the Related Art
In recent years, surface acoustic wave (hereinafter simply referred to as SAW) filters have come into wide use as filters on mobile telephones. Reasons of this wide use are that the SAW filters are small-sized, lightweight, and excellent in the shape factor, as compared to other filters such as a dielectric filter, a multilayered LC filter, or the like.
Generally, a SAW device is configured to use a SAW chip having comb-like interdigital transducers (hereinafter referred to as IDTs) on a piezoelectric material substrate (hereinafter referred to as piezoelectric substrate). The SAW chip is hermetically sealed within a cavity. In this configuration, electric signals are applied to an IDT on the input side, the signals are converted into the SAW, and the SAW travels on the piezoelectric substrate. It is thus possible to obtain the electric signals, on which a given modulation has been performed, from the other IDT on the output side.
FIG. 1 shows a configuration of a conventional SAW chip 900a. The SAW chip 900a is configured as a four-stage ladder-type filter.
Referring to FIG. 1, the SAW chip 900a includes an input signal pad 901, an output signal pad 902, ground pads 903, a dummy pad 904, interconnection patterns 905, a dicing line 906, short bars 907, parallel resonators 908, and series resonators 909. These are arranged on a main surface (which is an upper surface) of a piezoelectric substrate 923. More precisely, a parallel resonator 908 and a series resonator 909 are connected to the input signal pad 901 via an interconnection pattern 905. Another parallel resonator 908 and another series resonator 909 are connected to the output signal pad 902 via another interconnection pattern 905. Still another parallel resonator 908 is connected in parallel with the above-mentioned two series resonators 909 via still another interconnection pattern 905. The input signal pad 901 is used for feeding high-frequency signals to the SAW chip 900a. The output signal pad 902 is used for extracting the high-frequency signals that have passed the SAW chip 900a. Each of the parallel resonator 908 and the series resonator 909 is configured to include an IDT and reflection electrodes.
The parallel resonators 908 are respectively connected to ground pads 903 via interconnection patterns 905. The ground pads 903 serve as terminals that are connected to a ground line on a package. That is, the parallel resonator 908 is connected the series resonator 909 and the ground. A reference numeral 904 in FIG. 1 denotes a dummy pad.
The parallel resonator 908, the series resonator 909, the input signal pad 901, the output signal pad 902, the ground pads 903, and the dummy pad 904 are connected to a metal pattern provided for the purpose of avoiding pyroelectricity via the short bar 907. The metal pattern is arranged on an outer circumference of the upper side of the piezoelectric substrate 923, namely, the dicing line 906. The short bar 907 is also a metal pattern.
FIG. 2 shows a configuration of a conventional SAW chip 900b, which is configured as a DMS (Double Mode SAW) filter. Hereinafter, in FIG. 2, the same components and configurations as those of FIG. 1 have the same reference numerals.
Referring to FIG. 2, the SAW chip 900b includes a resonator 911 that is connected to the input signal pad 901 via the interconnection pattern 905, a three-IDT multimode filter 910 that is connected to the output signal pad 902 and three ground pads 903 via another interconnection pattern 905. These are arranged on the upper side of the piezoelectric substrate 923. The resonator 911 and the three-IDT multimode filter 910 are configured to include the IDTs and the reflection electrodes. The three-IDT multimode filter is configured to include three IDTs.
The resonator 911, the three-IDT multimode filter 910, the input signal pad 901, the output signal pad 902, the ground pads 903, and the dummy pads 904 are connected to the metal pattern that is provided on the dicing line 906 for the purpose of avoiding pyroelectricity, via the short bar 907.
The above-mentioned SAW chips 900a and 900b are respectively mounted on packages 900c as shown in FIG. 3 in a facedown state by a flip chip mounting. Mounting in the facedown state denotes that the upper side of the SAW chip, on which the IDTs and various pads are provided, is mounted with facing a mounting surface of the package 900c. 
As shown in FIG. 3, the package 900c includes the cavity so as to mount the SAW chip 900b on a substrate 924. An opening of the cavity is provided on a main surface, or the upper surface of the substrate 924. The bottom of the cavity is a chip mounting surface. On this mounting surface, an input signal interconnection 912, an output signal interconnection 913, and a ground interconnection 914 are arranged on positions that correspond to various pads of the SAW chip 900b. The above-mentioned interconnections are connected to the various pads with bumps when the SAW chip 900b is mounted, referring to bumps in FIG. 10. Thus, the package 900c and the SAW chip 900b are electrically coupled and are secured mechanically.
A backside input terminal 920, a backside output terminal 921, and a backside ground terminal 922, which serve as external terminals, are arranged on the backside of the package 900c. The backside input terminal 920 is electrically coupled to the input signal interconnection 912 on the mounting surface through the interconnection pattern including a via interconnection that penetrates a lower part of the substrate 924. The backside input terminal 921 is electrically coupled to the output signal interconnection 913 on the mounting surface through the interconnection pattern including another via interconnection that penetrates the lower part of the substrate 924. The backside ground terminal 922 is electrically coupled to the ground interconnection 914 on the mounting surface through the interconnection pattern including still another via interconnection that penetrates the lower part of the substrate 924. That is, the input signal interconnection 912, the output signal interconnection 913, and the ground interconnection 914 are electrically extracted onto the backside of the package 900c. 
A seal ring 915 is arranged on the circumference of the opening of the cavity on the substrate. The seal ring 915 is a member to firmly secure another substrate having a shape of plate that serves as a lid. That is, after the SAW chip 900b is mounted, the cavity is sealed with the lid substrate.
In the above-mentioned configuration, the interconnection patterns that connect the resonators were conventionally arranged on the SAW chip as shown in FIG. 1 or 2, for example, as described in Japanese Patent Application Publication No. 2000-332564 (hereinafter referred to as Document 1). Accordingly, any interconnection patterns that connect the resonators are not arranged on the package. In addition, in the conventional technique, it is common that the interconnection patterns are produced in the same process as other conductive patterns such as the resonators or the various pads.
In the case where the resonators and other conductive patterns such as the resonators or the various pads are produced in the same process, however, with respect to the film thickness, the interconnection patterns are as thin as the resonators. This caused a drawback in that a wiring resistance became great. This drawback brings a serious problem to a device used in a system with relatively high-frequency signals such as PCS (Personal Communications Services) particularly. This is because the filter for a high-frequency range has a thin resonator.
A filter for the 800 MHz band and a filter for the 1.9 GHz band may be produced with the identical materials and configurations. In the case where the filter for 800 MHz includes the resonator having a thickness of 330 nanometers, the filter for 1.9 GHz includes the resonator having a thickness of 140 nanometers. That is, the filter for the 1.9 GHz band is 0.4 times as thick as the filter for the 800 MHz band. As a result, a sheet resistance of the 1.9 GHz filter is 290 mΩ (milliohm), which is 2.6 times as much as that of the 800 MHz filter, which has the sheet resistance of 110 mΩ. Here, aluminum base alloy is employed for an electrode material.
As described above, as the wiring resistance becomes greater, the insertion loss becomes greater. Accordingly, there is a problem in that the filter characteristics are degraded. Additionally, this causes another problem in that as the insertion loss becomes greater, the power consumption is increased.
In the case where the interconnection patterns are produced in the same process as the resonator, the thickness of the interconnection pattern is determined by not only the characteristics required for the interconnection patterns but also other elements. Thus, there is still another problem in that it is difficult to adjust the impedance of the interconnection pattern at an arbitrary value.
In order to solve the above-mentioned problems, a width of the interconnection pattern may be greater, however, this makes a chip area greater. As a result, the SAW device will be greater, too.